Are there any good reasons to have a database table without an integer primary key?

Question

Although I\'m guilty of this crime, it seems to me there can\'t be any good reason for a table to not have an identity field primary key.

Pros: - whether you want to o

Answer 1

The most clear example of a table that doesn't need a surrogate key is a many-to-many relation:

CREATE TABLE Authorship (
  author_id INT NOT NULL,
  book_id   INT NOT NULL,
  PRIMARY KEY (author_id, book_id),
  FOREIGN KEY (author_id) REFERENCES Authors (author_id),
  FOREIGN KEY (book_id) REFERENCES Books (book_id)
);

I also prefer a natural key when I design a tagging system:

CREATE TABLE Tags (
  tag VARCHAR(20) PRIMARY KEY
);

CREATE TABLE ArticlesTagged (
  article_id INT NOT NULL,
  tag        VARCHAR(20) NOT NULL,
  PRIMARY KEY (article_id, tag),
  FOREIGN KEY (article_id) REFERENCES Articles (article_id),
  FOREIGN KEY (tag) REFERENCES Tags (tag)
);

This has some advantages over using a surrogate "tag_id" key:

• You can ensure tags are unique, without adding a superfluous UNIQUE constraint.
• You prevent two distinct tags from having the exact same spelling.
• Dependent tables that reference the tag already have the tag text; they don't need to join to Tags to get the text.